Mrs Clinton called for action to stop militants getting hold of assault rifles, saying weapons of war had no place on America's streets.

The investigation

FBI Director James Comey told reporters in Washington that there were "strong indications of radicalisation and of potential inspiration by foreign terrorist organisations".

"We're highly confident this killer was radicalised at least in some part through the internet," he said.

In phone calls to the authorities from the nightclub, Mateen said he was carrying out the attack for IS but he also pledged allegiance to a suicide bomber for the al-Nusra Front group in Syria, and to the perpetrators of the Boston Marathon bombing, none of whom is linked to IS.

"We see no indication that this was a plot directed from outside the United States, and we see no indication that he was part of any kind of network," Mr Comey said.

Investigators are going through the killer's life, and especially his electronic devices, to try to understand his motivation, Mr Comey said.